The ribosomal DNA (rDNA) from the yeast Saccharomyces cerevisiae is an interesting and versatile system for studying gene structure and expression, chromosome organization and chromosome replication in a simple eucaryote. A basic knowledge of the eucaryotic cellular regulatory mechanisms which govern gene expression and DNA replication is essential in understanding normal cell growth and alterations which occur in pathologic states such as infection, cancer, inherited metabolic errors, etc. Our experiments concern specific aspects of these topics and are divided into three main sections. The first section includes experiments which will examine the chromosomal location and the organization of the reiterated ribosomal RNA (rRNA) genes both genetically and biochemically. The discovery of a yeast strain which contains rDNA with an EcoRI restriction endonuclease pattern different from that usually found enables us to follow the segregation of rDNA in a genetic cross and to examine its linkage to other genetic markers. Biochemical experiments will include analysis of high molecular weight DNA containing rDNA by electron microscopy and restriction endonuclease digestion, as well as the cloning and characterization of DNA adjacent to rDNA. The second section involves an examination of the fine structure of individual rDNA repeating units from different yeasts. By noting the degree and pattern of evolutionary divergence among these rDNAs, we expect to gain information regarding the functional roles of different parts of the rDNA units. The final section of our experimental plan includes experiments to determine the timing of rDNA replication during the yeast cell cycle and to examine the fine structure of replicating chromosomes which contain rDNA.